spring boot 自动装配的原理

参考： https://blog.csdn.net/Dongguabai/article/details/80865599。如有侵权，请联系本人删除！

入口：

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@SpringBootApplication

@EnableEurekaServer

public class SpringCloundEurekaServerApplication {

    public static void main(String[] args) {

        SpringApplication.run(SpringCloundEurekaServerApplication.class, args);

    }

这里有个很重要的注解：@SpringBootApplication。这是一个组合注解，点击进入可以看到详情如下：

@Target(ElementType.TYPE)                 //作用于类上面

@Retention(RetentionPolicy.RUNTIME)       //作用域运行时

@Documented                               //doc文档

@Inherited                                //被继承

@SpringBootConfiguration

@EnableAutoConfiguration

@ComponentScan(excludeFilters = {

        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

public @interface SpringBootApplication {

......

}

上面的组合注解也太多特别的，前面4个注解，就是4个元注解而已。主要关注其中的两个标红的注解。

先分析@SpringBootConfiguration，这也是一个组合注解，点击进去看到详情如下：

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Configuration

public @interface SpringBootConfiguration {

}

从上面可以看出，也就是组合了@Configuration，标志是配置类而已。

再看看@EnableAutoConfiguration，点击进入看到详情是：

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**

     * Exclude specific auto-configuration classes such that they will never be applied.

     * @return the classes to exclude

     */

    Class<?>[] exclude() default {};

    /**

     * Exclude specific auto-configuration class names such that they will never be

     * applied.

     * @return the class names to exclude

     * @since 1.3.0

     */

    String[] excludeName() default {};

}

其中，@AutoConfigurationPackage，这个注解组合情况如下：

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@Import(AutoConfigurationPackages.Registrar.class)

public @interface AutoConfigurationPackage {

}

AutoConfigurationPackages.Registrar.class 这个类很关键。在我以前的博客Spring的@Enable*注解的工作原理，也写到，关于@Enablexxx 注解的三种实现，其中一种就是用到了这个类似于Registrar.class的

类。该类重写了两个方法，如下：

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override

        public void registerBeanDefinitions(AnnotationMetadata metadata,            //spring初始化的时候，这个BeandEfinitions用得非常多

                BeanDefinitionRegistry registry) {

            register(registry, new PackageImport(metadata).getPackageName());

        }

        @Override

        public Set<Object> determineImports(AnnotationMetadata metadata) {

            return Collections.singleton(new PackageImport(metadata));

        }

    }

单纯从代码还看不出什么，debug的时候，可以看到这个metadata 就是一个我们的启动类，截图如下：

 也就是说，这个注解的作用，就是扫描com.drafire.springcloundeurekaserver.SpringCloundEurekaServerApplication 启动类所在的包以及子包，并注入到spring容器中
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下面再看下：AutoConfigurationImportSelector.class，源码如下：

、public class AutoConfigurationImportSelector

        implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,

        BeanFactoryAware, EnvironmentAware, Ordered {

    private static final AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationEntry();

    private static final String[] NO_IMPORTS = {};

    private static final Log logger = LogFactory

            .getLog(AutoConfigurationImportSelector.class);

    private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";

    private ConfigurableListableBeanFactory beanFactory;

    private Environment environment;

    private ClassLoader beanClassLoader;

    private ResourceLoader resourceLoader;

    @Override

    public String[] selectImports(AnnotationMetadata annotationMetadata) {

        if (!isEnabled(annotationMetadata)) {

            return NO_IMPORTS;

        }

        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader

                .loadMetadata(this.beanClassLoader);

        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(

                autoConfigurationMetadata, annotationMetadata);

        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

    }

    /**

     * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}

     * of the importing {@link Configuration @Configuration} class.

     * @param autoConfigurationMetadata the auto-configuration metadata

     * @param annotationMetadata the annotation metadata of the configuration class

     * @return the auto-configurations that should be imported

     */

    protected AutoConfigurationEntry getAutoConfigurationEntry(

            AutoConfigurationMetadata autoConfigurationMetadata,

            AnnotationMetadata annotationMetadata) {

        if (!isEnabled(annotationMetadata)) {

            return EMPTY_ENTRY;

        }

        AnnotationAttributes attributes = getAttributes(annotationMetadata);

        List<String> configurations = getCandidateConfigurations(annotationMetadata,

                attributes);

        configurations = removeDuplicates(configurations);

        Set<String> exclusions = getExclusions(annotationMetadata, attributes);

        checkExcludedClasses(configurations, exclusions);

        configurations.removeAll(exclusions);

        configurations = filter(configurations, autoConfigurationMetadata);

        fireAutoConfigurationImportEvents(configurations, exclusions);

        return new AutoConfigurationEntry(configurations, exclusions);

    }

.........

}

这里有个非常重要的重写方法：selectImports，其实这个也是@Enablexxx的三种实现方法之一，具体参考我以前的博客Spring的@Enable*注解的工作原理。

一步一步跟踪，可以看到，最终是调用 getCandidateConfigurations(annotationMetadata,attributes)，跟踪进入可以看到，源代码如下：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,

            AnnotationAttributes attributes) {

        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(

                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());

        Assert.notEmpty(configurations,

                "No auto configuration classes found in META-INF/spring.factories. If you "

                        + "are using a custom packaging, make sure that file is correct.");

        return configurations;

    }

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {

        String factoryClassName = factoryClass.getName();

        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

    }

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);

        if (result != null) {

            return result;

        } else {

            try {

                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");

                LinkedMultiValueMap result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {

                    URL url = (URL)urls.nextElement();

                    UrlResource resource = new UrlResource(url);

                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);

                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {

                        Entry<?, ?> entry = (Entry)var6.next();

                        String factoryClassName = ((String)entry.getKey()).trim();

                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());

                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {

                            String factoryName = var9[var11];

                            result.add(factoryClassName, factoryName.trim());

                        }

                    }

                }

                cache.put(classLoader, result);

                return result;

            } catch (IOException var13) {

                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);

            }

        }

    }

从上面明显可以看出，classLoader最终是从 META-INF/spring.factories 中循环读取各种配置，并注入到spring 容器中。META-INF/spring.factories 在 spring-boot-autoconfigure-1.5.19.RELEASE.jar（各个版本号不一样）。看看spring.factories的详细内容，如下：

# Initializers

org.springframework.context.ApplicationContextInitializer=\

org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\

org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# Application Listeners

org.springframework.context.ApplicationListener=\

org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners

org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\

org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters

org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\

org.springframework.boot.autoconfigure.condition.OnClassCondition

# Auto Configure

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\

org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\

org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\

org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\

org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\

org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\

org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\

org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\

org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\

org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\

org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\

org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\

org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\

org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\

org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\

org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\

org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\

org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\

org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\

org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\

org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\

org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\

org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\

org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\

org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\

org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\

org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\

org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\

org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\

org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\

org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\

org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\

org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\

org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\

org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\

org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\

org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\

org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\

org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\

org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\

org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\

org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\

org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\

org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\

org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\

org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\

org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\

org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\

org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\

org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\

org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\

org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\

org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\

org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\

org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\

org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\

org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\

org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\

org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\

org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\

org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\

org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\

org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\

org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\

org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\

org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\

org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\

org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\

org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\

org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\

org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\

org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\

org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\

org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\

org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\

org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\

org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\

org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\

org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\

org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\

org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\

org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

# Failure analyzers

org.springframework.boot.diagnostics.FailureAnalyzer=\

org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\

org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\

org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer

# Template availability providers

org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\

org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\

org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\

org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\

org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\

org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

上面不仅仅是字符串，其实都是各个配置类的链接来的，可以按住ctrl+鼠标左键进入查看该配置类的详情。

这里面的配置类，包含很多，如果redis、rabbitmq、webmvc、OnClassCondition 等。通过这个配置，自动导入很多配置并生效。



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回顾整体流程，Springboot的启动，主要创建了配置环境(environment)、事件监听(listeners)、应用上下文(applicationContext)，并基于以上条件，
在容器中开始实例化我们需要的Bean，至此，通过SpringBoot启动的程序已经构造完成
而自动注入，则是通过Import(AutoConfigurationImportSelector.class)注解，通过一系列的操作，包括loadClassName、loadFactoryNames，将META-INF/spring.factories 的类信息，加载到容器（一个ConcurrentHashMap，以BeanDefineTion的封装形式）里面